The present invention relates to an optical fiber cable including an optical fiber ribbon stack. In particular, the present invention relates to an optical fiber cable including an optical fiber ribbon stack and cushion members disposed on sides of the optical fiber ribbon stack for protecting corner fibers of the optical fiber ribbon stack from excessive bending and contact stresses.
Optical fiber cables are used to transmit information at very high rates over long distances. Buffer tubes are typically utilized in optical fiber cables as the primary structure for protecting the optical fiber units contained within. In particular, the buffer tubes typically house an optical unit such as one or more loose optical fibers or an optical fiber ribbon stack having a plurality of optical fibers held together in a planar array by a common matrix material.
In a loose tube cable structure, a plurality of buffer tubes, each housing one or more loose optical fibers or an optical fiber ribbon stack, are stranded around a central strength member to form a stranded core which is jacketed with an additional protective layer. In a monotube cable structure, a plurality of optical fibers or an optical fiber ribbon stack are housed in a single, centrally located buffer tube which is jacketed with an additional protective layer. Further, reinforcing yarns or fibers as well as water blocking materials in the form of gels or hot melts, water swellable powders, yarns or tapes, and/or corrugated armor may be utilized between the jacket and the buffer tubes.
In a slotted core cable, optical fiber ribbon stacks are disposed in slots or grooves formed in an exterior surface of a central elongate rod-like member. Alternatively, the optical fiber ribbon stacks may be housed in buffer tubes which are disposed in the slots. In either case, the optical fiber ribbon stacks or buffer tubes housing the optical fiber ribbon stacks are held in the slots by outer sheath or binding tape which surrounds the slotted rod-like member.
The buffer tubes housing the optical fibers may be left empty or may be filled with a water blocking compound such as a thixotropic gel which prevents water ingress but allows for fiber movement during cable expansion or contraction or under mechanical loads. It is also known to use water swellable or superabsorbent materials, in the form of tape, powder or yarn, which absorb water.
A problem in the design of optical cables employing ribbon stacks is attenuation in corner fibers of the ribbon stacks which may reduce overall performance characteristics of the cables and place limitations on the level of recommended thermo-mechanical loads. As shown in FIG. 1, the corner fibers 15 of a ribbon stack 10 are located furthest from the geometrical center 14 of the fiber ribbon stack 10. Consequently, the corner fibers 15 are subjected to maximum stresses under bending conditions and contact stresses from surrounding components such as a buffer tube 20 housing the ribbon stack 10 and an outer jacket (not shown) surrounding the buffer tube 20, in central tube single-ribbon configurations. That is, in the case of bending with respect to the shown bending axis A, the corner fibers 15 are subjected to maximum stresses resulting in fiber deformation and consequently, attenuation of the corner fibers 15. It is believed that attenuation is caused by the reduction in the radius of the fiber curvature.
One of the criterion utilized to predict performance of an optical fiber cable is the stress state of the corner fibers. Another criterion is the tensile and compressive windows showing how much displacement under tension and thermal contraction is allowed for the fibers inside the buffer tube before the fiber hits the buffer tube wall. In particular, design for cables including ribbon stacks is commonly focused on providing a sufficiently large gap or separation distance between the corners of the fiber stack and other structural members such as the wall of the buffer tube in order to avoid or delay contact and bending of the corner fibers which lead to the attenuation problems. Therefore, in conventional cable designs which attempt to prevent possible attenuation problems, the diameter of the buffer tube containing the ribbon stacks may be increased resulting in an undesirable increase in the overall cable diameter.
In view of the disadvantages and problems associated with housing ribbon stacks in buffer tubes, it is an object of the present invention to provide an optical cable wherein the corner fibers of a ribbon stack are protected from excessive stresses in order to improve overall performance and load carrying capacity.
The present invention is adapted to achieve the foregoing objects. In accomplishing these objects, the present invention provides an optical fiber cable comprising an optical fiber ribbon stack including a plurality of optical fibers, and a plurality of cushion members disposed on outer side surfaces of the optical fiber ribbon stack. The cushion members function as spacers and strain energy absorbing members for protecting corner fibers of the optical fiber ribbon stack from bending and contact stresses.
The cushion members have material characteristics, such as contact hardness and Young""s modulus, which are similar to those of the ribbon stack, in particular, ribbon matrix. Further, the material characteristics of the cushion members may be graded or gradually change from a soft inner layer at the sides of the cushion members which contact the ribbon stack to a stiff outer layer at the sides of the cushion which may contact the buffer tube or other surrounding elements. When the cushion members contact the sides of the buffer tube under crushing and transverse impact loading conditions, the cushion members are deformed thereby effectively absorbing strains and stresses and protecting the corner fibers of the ribbon stack.
According to the present invention, there is further provided an elastic membrane surrounding the optical fiber ribbon stack and attached cushion members, and a filler material disposed in the space between the ribbon stack and the elastic membrane and the space between the elastic membrane and the buffer tube. The filler material may include a soft polymer, as well as bubbled foam or foamed gel with or without reused materials such as chips or powder of a low-density polyethylene and other non-metals and metals. The elastic membrane and the filler material provide a surface for permitting the sliding of the components in the longitudinal direction of the optical cable so that less strain energy is transmitted from the buffer tubes to the fibers under thermally induced contraction, longitudinal bending or buckling of fibers.
The above and other features of the invention including various and novel details of construction and process steps will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular optical cable structure embodying the invention is shown by way of illustration only and not as a limitation of the invention. The principles and features of this invention may be employed in varied and numerous embodiments without departing from the scope of the invention.